1. Field of the Invention
The invention relates to the art of image rendering. It finds application where color images are rendered with a single colorant. For example, the invention finds application in repro-graphic machines where a color image is scanned and then transformed for rendering so that black and white copies of the color image can be made. The invention finds further application in general-purpose computing devices such as personal computers and business graphics authoring devices. In the latter devices, color images, such as, for example, bar and pie charts are created in color and displayed on a computer display. Thereafter, single colorant versions of the color images may be printed on, for example, black and white printers.
2. Description of Related Art
Communicating concepts and ideas to others can be difficult. One method often used to communicate ideas to a group of people is to make a visual presentation. In a visual presentation, images such as charts, graphs and photographs are often displayed before an audience while a speaker explains and describes the significance of the images. Alternatively, the images can act as summaries of an associated speech. Typically, the images are presented in color. Color often adds to the impact and clarity of an image. For example, a pie chart or a bar graph is easier to read if its various sections are presented in different colors.
Copies of visual presentation material are often distributed to the audience members. The distributed materials serve to document the presentation. The distributed material can help audience members follow the presentation and can serve as a study aid or reference material.
Unfortunately, it can be impractical or prohibitively expensive to distribute a large number of color copies of presentation material. Furthermore, in some cases, color reproduction equipment is not readily available. In these cases the color images are often reproduced in black and white.
Creating black and white versions of color images can be problematic. Typically, a great deal of information is lost in the conversion to black and white. For example, typical color image authoring devices can produce over sixteen million different colors, while typical black and white rendering devices can only produce two hundred fifty six shades of gray. Obviously, a great number of colors must be mapped to each level of gray. Therefore, portions of a color image that are quite obviously different colors can appear to be the same color when the image is rendered in black and white. When the image portions in question are, for example, different sections of a pie chart or bar graph, this loss of information can render the chart or graph useless.
Attempts have been made to alleviate this problem by using texturing to increase the number of ways colors can be represented in a black and white image. For example, one texturing or patterning technique is described in U.S. Pat. No. 4,903,048 to Harrington. Typically, under these strategies, the conversion to black and white is accomplished by dividing a color space into a finite number of bins and assigning a different halftone pattern to each of the bins. This approach does preserve more information from the color image. However, this approach can lead to abrupt transitions in the black and white image, which may not be desired in some applications. In this regard, where colors in the original image smoothly blend from one color to another, the blend in color can cross a bin boundary, resulting in a sudden shift in a halftone pattern or level. This situation can be further aggravated by the presence of noise in the image. For example, a subtle jitter or shift in the color in a photograph of a persons face can be transformed into dramatic changes in halftone patterns if the jitter or shift is across one or more bin boundaries.
All halftoning methods by definition introduce some distortion in the output image. Designers of halftoning methods typically make tradeoffs in representing the visual parameters of spatial detail versus tonal fidelity. The additional requirement to represent color statically in the halftone pattern reduces the capability of the halftoning system to represent the two visual parameters. For this reason, where these techniques are used, they are generally not available in “walk up mode”. Therefore, prior art image processors often have controls for various modes of operation which set tradeoffs between the visual parameters and the addition texture related features. The use of these controls puts an additional cognitive load on the “walk up” or casual users. The user must know the features are available and know how to use them. Thus it is a great advantage if the image processor can make intelligent choices on when and where to make the tradeoff of color representation. A method of generating black and white versions of color images is needed that preserves as much information as possible from an original color image, while minimizing the amount of distortion introduced into the black and white image. Furthermore, a method is needed that is invoked automatically when a particular image requires the use of the method, and then, only at points in the image that require it, thus providing “walk up mode” availability of the method.